It's in the name Old Faithful, the geyser in Yellowstone National Park that blows over a hundred feet in the air every 90 minutes or so. It's enchanted millions of visitors for generations.

ROBERT SIEGEL, HOST:

Geophysicist Robert Smith has been mesmerized for 61 years, since before he started college. And he has long wondered something.

ROBERT SMITH: What people really did not understand - and we didn't of course - is where is the reservoir that holds the water, the steam, the gas that fuels Old Faithful.

SIEGEL: Well, he has now coauthored a study that appears in Geophysical Research Letters based on research that he did with his team.

SMITH: We put out seismographs on the ground, up to 150 or so. And we record the - just the natural ground vibrations.

MCEVERS: From those, they created an image of what's below the surface - a reservoir that fills up with gases and water. Smith says picture something in your kitchen.

SMITH: You fill a teakettle up with cold water and put it on the stove. And as the teakettle heats, it takes so many minutes. And then finally, when the pressure from the steam and hot water is big enough, it pops open the teakettle nozzle, if you wish, and out comes the steam.

SIEGEL: In Old Faithful's case, it takes about 90 minutes to refill, and then it blows. Sixty-one years after first spending a summer study in Yellowstone's tributaries, Robert Smith still loves the thrill of discovery.

SMITH: You know, a lot of the public thinks about geology as boring, billion-year-old rocks. Well, Yellowstone is anything but.